Belgium, because there are innate problems with the structure of the country.
Flanders should join the Netherlands and Wallonia should join France and BHV (or perhaps Brabant) should be its own country.
They have far more common with each other than where you want to put them. They'd get along even worse with their new partners.
Well, that could certainly be true of the Flemish toward Dutch. Those conservative Catholics wouldn't have so much in common with the liberal and/or tolerant and/or open-minded (choose what define the Dutch society the best) Dutch.
But for French and Wallons, I beg to differ. Culturally they are quite close, and the cultural ties have been regularly cultivated along the years, for example a lot of popular singers in France came from Belgium, and song is a very popular thing in every sense of the term, but in other fields too, humor, writing, TV, lots of cultural exchanges, and a big cultural French speaking Belgian presence in France. French speaking Belgians have French TV too, and you can regularly find Belgians in TV shows such as TV games and other stuffs. On Internet, you regularly find Belgians on the 'French sites', Québequois too, but French and Belgians feel closer than French and Québequois.
Ok, there is a kind of traditional cultural contempt from France to Belgium, but that isn't a deep one at all, that is more a Americans/Canadians stuffs, that are our Canadians, and we're their arrogant Americans. We have jokes on Belgians being so stupid, and they have jokes on us being so arrogant, and so it goes, nothing very deep, and rather old-school moreover I'd say.
Actually there would be less differences between most French and Wallons than between most French and Alsatians, or even less differences between people living in the north and the east of France with Wallons, than between those French and those living in south-east.